The present invention relates to a radio system which may be used for digital automobile telephones, personal handy phones, etc.
FIG. 1 is a block diagram showing the arrangement of a conventional radio system for digital automobile telephones or the like.
In the radio system arranged as shown in FIG. 1, a signal received through an antenna 1 is separated by an antenna multiplexer 2 and then sent to a radio-frequency amplifier 3 where it is amplified.
The amplified signal is supplied to a band-pass filter 4 where undesired waves are removed, and then down-converted in a mixer 5 with a first local frequency. The converted signal is sent to a band-pass filter 6 of first intermediate frequency where undesired waves are removed to produce a first intermediate-frequency signal, which is then amplified in a first intermediate-frequency amplifier 7.
Further, the first intermediate-frequency signal is down-converted in a mixer 8 with the output frequency of a receiving second local oscillator 13 to produce a second intermediate-frequency signal, which is then supplied to a band-pass filter 9 of second intermediate frequency where undesired waves are removed. Thereafter, the second intermediate-frequency signal is amplified in a second intermediate-frequency amplifier 10, detected in a detector 11, and demodulated in a baseband signal processor 12.
On the other hand, a signal to be transmitted which is generated in the baseband signal processor 12 is quadrature modulated in a quadrature modulator 24 with the frequency of an output signal from a transmitting second local oscillator 23 and then supplied to a band-pass filter 22 where undesired waves are removed.
The signal, which has undesired waves removed therefrom, is up-converted with the first local frequency in a mixer 21 and then supplied to a band-pass filter 20 where undesired waves are removed. Thereafter, the signal is amplified to a normal power level in a power amplifier 19.
The signal, which has been amplified to the normal power level, is supplied to the antenna 1 via the antenna multiplexer 2 and then transmitted.
The conventional radio system, arranged as described above, suffers from the problem that the transmit and receive frequencies are higher than those of analog automobile telephones, cordless telephones, etc., resulting in an increase in the power consumed in a PLL frequency synthesizer 15, a fixed oscillator 18 and other elements, which are used to constitute a local oscillator. The increase in the power consumption is a particularly serious problem in mobile telephones, which use a battery as a power source.
The problem of the conventional radio system will be explained below by way of an example of preset frequencies for digital automobile telephones operating in the 1.5 GHz band.
In communication between a base station and a mobile station, the transmit frequency used at the mobile station is specified as falling in the range of 1,429 to 1,441 MHz or in the range of 1,453 to 1,465 MHz, and the receive frequency in the range of 1,477 to 1,489 MHz or in the range of 1,501 to 1,513 MHz.
Let us take the transmit frequency band of 1,429 to 1,441 MHz and the receive frequency band of 1,477 to 1,489 MHz by way of example for the following explanation. Assuming that the first and second intermediate frequencies are 130 MHz and 450 KHz, respectively, which are common in the 800 MHz band, the first local frequency is 1,607 to 1,619 MHz {(1,477+130) MHz to (1489+130) MHz}, and this is the frequency of the PLL frequency synthesizer 15.
The output frequency of the receiving second oscillator 13 is 129.55 MHz (130 MHz-450 KHz).
In addition, from the relationship between the first local frequency (1,607 to 1,619 MHz) and the transmit frequency (1,429 to 1,441 MHz), the transmitting second local frequency is 178 MHz (1,607-1,429 MHz). The output frequency of the fixed oscillator 18 must approach the frequency of the PLL frequency synthesizer 15. Therefore, if it is determined to be 12 times the receiving second local frequency, the output frequency of the fixed oscillator 18 is 1,554.6 MHz.